Photoelectrophoretic imaging process employing a layer of insulating liquid to improve image quality



Dec. 23, 1969 1.. M. CARREIRA 3,485,738

PHOTOELECTROPHORETIC IMAGING PROCESS EMPLOYING A LAYER OF INSULATING LIQUID TO IMPROVE IMAGE QUALITY Filed Jan. 6, 1966 INVENTOR. LEONARD N. CARRE IRA A TTOR/VEVS United States Patent O 3,485,738 PHOTOELECTROPHOERETIC Iii AGING PROCESS EMPLOYTNG A LAYER OF INSULATING LIQUID TO IMPROVE IMAGE QUALITY Leonard M. Carreira, Webster, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Jan. 6, 1966, Ser. No. 519,034 Int. Cl. B012: 5/02; B64g l/30; F151) 15/00 "US. Cl. 204-181 5 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for improving electrophoretic imaging by filling the space between the electrodes of the electrophoretic imaging system with an insulating liquid, then contacting the suspension with the electrode and applying an electric field for imaging.

This invention relates in general to imaging systems and, more' specifically, to an improved electrophoretic imaging system.

There has been recently developed an electrophoretic imaging system capable of producing color images which utilizes electrically photosensitive particles. This process is described in detail and claimed in Patents Nos. 3,384,- 565; 3,384,566; and 3,383,993 all issued on May 21, 1968. In such an imaging system, variously colored light absorbing particles are suspended in a non-conductive liquid carrier. The suspension is placed between electrodes, subjected to a potential difference and exposed to an image. As these steps are completed, selected particle migration takes place in image configuration, providing a visible image at one or both of the electrodes. An essential component of the suspended particles which must be electrically photosensitive and which apparently undergo a net change in charge polarity upon exposure to activating electromagnetic radiation, through interaction with one of the electrodes. In a monochromatic system, particles of a single color are used, producing a single colored image equivalent to conventional blackand-white photography. In a polychromatic system, the images are produced in natural color because mixtures of particles of two or more different colors which are each sensitive only to light of a specific wave-length or narrow range of wave-lengths are used. Particles used in this system must have both intense and pure colors and be highly photosensitive.

Ordinarily, electrophoretic imaging systems include a transparent, conductive injecting electrode upon which the dispersion of photosensitive particles in an insulating liquid is coated. The image to be reproduced is projected on the suspension to the injecting electrode. During exposure, a potential, usually of from 300 to 2,000 volts is imposed on the suspension between the injecting electrode and a relatively insulating blocking electrode. This blocking electrode, ordinarily in the form of a roller or an endless belt, consists of a conductive core with an insulating surface. This blocking electrode is passed across the surface of the liquid suspension during exposure. Unwanted photosensitive particles migrate to the surface of the blocking electrode, leaving an image on the injecting electrode corresponding to the original.

It has often been found that images produced by the system broadly described above have uneven density and are blotchy or mottled in appearance. It is theorized that the unevenness in the image is caused by varying corona discharge or air ionization between the blocking electrode and the injecting electrode as the blocking electrode approaches the particle suspension. While the system described above is often capable of producing excellent images, at times, especially during periods of high relative humidity the images produced are not of commercially acceptable quality. Thus, there is a continuing need for improvements in image quality under all ambient conditions.

It is, therefore, an object of this invention to provide an electrophoretic imaging system which is devoid of the above noted disadvantages.

It is another object of this invention to provide a method of eliminating varying corona discharge or air ionization between electrodes in electrophoretic imaging systems.

It is another object of this invention to provide an electrophoretic imaging system capable of producing images of uniform density under varied humidity conditions.

The foregoing objects and others are accomplished in accordance with this invention by filling the air gap between the electrodes in an electrophoretic imaging system with an insulating liquid before the electrodes are brought into proximity in the image forming area. The insulating liquid may comprise any material which is sufiiciently insulating to prevent corona discharge between the electrodes and which is compatible with the insulating liquid in which the imaging particles are suspended. Preferably, the liquid should have a resistivity of about 10 ohm/cm. or greater. It is also preferred that the liquid have the same general composition as the carrier liquid which the photosensitive particles are suspended to insure compatibility should some of this liquid be carried by the roller into the imaging area. Suitable insulating liquids include Sohio Odorless Solvent 3440 (a kerosene fraction available from Standard Oil of Ohio), Isopar G (a long chain saturated aliphatic hydrocarbon available from Humble Oil Company of New Jersey), Freon (a fiuorinated hydrocarbon available from E. I. du Pont de Nemours and Company), Florochemical FC- (a poly-fiuorinated mixture of compounds containing eight carbon atoms, available from Minnesota Mining and Manufacturing Co.), Silicone Fluid SF-96 (a dimethyl silicane fluid available from General Electric), mineral oil, decane, dodecane, N-tetradecane, molten paraifin, molten beeswax or other molten thermoplastic material. Any other suitable insulating liquid may be used where desired.

The advantages of this improved electrophoretic imaging system will become further apparent upon consideration of the following detailed disclosure of the invention; especially when taken in conjunction with the accompanying drawing which shows a side view of a simple exemplary for carrying out the process of this invention.

Referring now to the figure, there is seen a transparent electrode generally designated 1 which, in this exemplary instance, is made up of a layer of optically transparent glass 2 overcoated with a thin optically transparent layer 3 of tin oxide, commercially available under the name NESA glass. This electrode shall hereinafter be referred to as the injecting electrode. Coated on the surface of injecting electrode 1 is a thin layer 4 of finely divided photosensitive particles dispersed in an insulating liquid carrier. The term photosensitive for the purpose of this invention, refers to the properties of a particle which, once attracted to the injecting electrode, will migrate away from it under the influence of an applied electric field when it is exposed to actinic electromagnetic radiation. For a detailed theoretical explanation of the apparent mechanism of operation of this imaging process, see the above mentioned Patents Nos. 3,384,565; 3,384,566; and 3,383,993, the disclosures of which are incorporated herein by reference. Adjacent to the liquid suspension 4 is a second electrode 5 hereinafter called the blocking electrode, which is connected to one side of the potential source 6 through a switch 7. The opposite side of potential source 6 is connected to the injecting electrode 1 so that when switch 7 is closed, an electric field is applied across the liquid suspension 4 between electrodes 1 and as blocking electrode 5 passes over liquid suspension 4. An image projection made up of light source 8, a transparency 9, and a lens 10 is provided to expose the dispersion 4 to a light image of the original transparency 9 to be reproduced. Electrode 5 is made in the form of a roller having a conductive central core 11 connected to the potential source 6. The core is covered with a layer of a blocking electrode material 12, which may be Baryta paper or other suitable insulating material. The blocking electrode may, of course, be in any other configuration capable of contacting the suspension wi hout relative movement at the suspension surface. Typical alternative configurations are described in copending application Ser. No. 452,651, filed May 3, 1965. The particle suspension is exposed to the image to be reproduced while potential is applied across the blocking and injecting electrodes by closing switch 7. Roller 5 is caused to roll across the top surface of injecting electrode 1 with switch 7 closed during the period of image exposure. This light exposure causes exposed particles originally attracted to electrode 1 to migrate through the liquid and adhere to the surface of the blocking electrode 5, leaving behind a pigment image on the injecting electrode surface which is a duplicate of the original transparency 9. After exposure, the relatively volatile carrier liquid evaporates off", leaving behind the image. This process, utilizing only components exemplified by those discussed above, is in itself, capable of ordinarily producing good images. However, under certain circumstances, such as high humidity, the image produced tends to have variable density and a mottled or blotchy appearance. It has been found that this problem may be overcome by providing an applicator means 13 to apply an insulating liquid to the surface of blocking electrode 5 which flows across the surface of the blocking electrode forming a pool at 14 which fills the gap between blocking electrode 5 and injecting electrode 1 until blocking electrode 5 reaches dispersion 4. The gap-filling insulating liquid may be applied by any convenient method. For example, the liquid may be applied by spray or dropper to the blocking electrode surface or to the injecting electrode surface at a location so as to fill the space between the electrodes. Where the particle suspension is, alternatively, coated onto the roller blocking electrode instead of the injecting electrode, the corona-preventing liquid may be applied to the injecting electrode, either by moistening the entire surface of the injecting electrode or by form ing a pool at the gap between the electrodes. It appears that this gap filling insulating liquid prevents corona discharge or air ionization between electrodes 5 and 1 and thus eliminates disruption of the image.

Any suitable insulating liquid may be used as the carrier for the photosensitive particles in this system. Typical carrier materials are decane, dodecane, N-tetradecane, parafiin, beeswax or other thermoplastic materials, Sohio Odorless Solvent 3440 (a kerosene fraction available from Standard Oil Company of Ohio) and Isopar-G (a long chain saturated aliphatic hydrocarbon available from Humble Oil Company of New Jersey).

As discussed above, any suitable insulating liquid may be used to fill the gap between the injecting and blocking electrodes. Good quality images have been produced with voltages ranging from 300 to 5,000 volts, in the apparatus of the figure.

The following examples further specifically define the present invention with respect to the use of an insulating liquid between the electrodes before imaging in an electrophoretic imaging system. Parts and percentages are by weight unless otherwise indicated. The examples below are intended to illustrate various preferred embodiments of the present invention.

All of the following examples are carried out in an apparatus of the general type illustrated in the figure with the particle mix 4 coated on a NESA glass substrate through which exposure is made. The NESA glass is connected in series with a switch, a potential source, and the conductive center of a roller having a coating of Baryta paper on its surface. The roller is approximately 2 /2 inches in diameter and is moved across the plate surface at about 1.45 centimeters per second. The plate employed is roughly 3 inches square and is exposed to a light intensity of about 8,000 foot candles as measured onthe uncoated NEST glass surface. The suspension is exposed to an image by means of a conventional transparency. Examples I-V use a black-and-White transparency and produce monochromatic images. Examples VI-X use a Kodachrome transparency and produce full color images. All pigments which have a relatively large particle size as received commercially or as made are ground in a ball mill for 48 hours to reduce their size to provide a more stable dispersion which improves the resolution of the final images.

EXAMPLE I About 8 parts of 2,4,6-tris(3-pyrenylazo) phloroglucinol is mixed with about 100 parts Sohio Odorless Solvent 3440. This dispersion is coated onto the NESA glass substrate. No liquid is introduced into the gap between the roller electrode and the NESA glass substrate, leaving a small air gap therebetween. Relative humidity is measured and is found to be about 35 percent. A potential of about 2500 volts is imposed on the roller electrode during exposure. After exposure, an image corresponding to the original is seen on the NESA surface and a negative image is seen on the Baryta surface of the roller electrode. The positive image is of excellent quality but with slight variations in density across the image surface.

EXAMPLE II The imaging process of Example I is repeated, except that sulficient Sohio Odorless Solvent 3440 is applied to the roller electrode before imaging to fill the air gap bv' tween the roller electrode and the NESA glass substrate before the potential is applied between the electrodes. After exposure, an excellent image corresponding to the original is observed on the NESA surface. Resolution and contrast are equal to that produced in Example I and image density is somewhat more uniform across the image surface.

EXAMPLE III The imaging process of Example I is repeated, however, the relative humidity is maintained at about percent. An image of good quality but with severe variations in density and a mottled and blotchy apperance results.

EXAMPLE IV The imaging process of Example III is repeated, except that sufficient Sohio Odorless Solvent 3440 is applied to the surface so as to fill the air gap between the roller electrode and the NESA surface before potential is applied across said electrodes. The image produced is of excellent quality with much improved image density uniformity.

EXAMPLE V The imaging process of Example IV is repeated, except Isopar-G is applied to the blocking electrode surface instead of the Sohio Odorless Solvent 3440. The image produced is of excellent quality with very uniform density across the image surface.

EXAMPLE VI A pigment mix is prepared comprising equal parts of a yellow pigment, Algol Yellow GC, l,2,5,6-di(C,C'-diphenyl)-pyrazol-anthraquinone, 'C.I. No. 67300, available from General Dyestuffs; a magenta pigment, Locarno 5 Red X-1686, 1-(4'-methyl-5-phloroazobenzene 2' sulfonic acid)-2-hydroxy-3-naphthoie acid, C.I. No. 15865, available from American Cyanamid; and a cyan pigment, Monolite Fast Blue GS, a mixture of alpha and beta forms of metal-free phthalocyanine, available from Arnold Hoffman Company. About 8 parts of this mixture is dispersed in about 100 parts Isopar-G and the suspension is coated onto the NESA glass substrate. No liquid is applied to the roller electrode surface, leaving an air gap between the roller electrode and the NESA glass substrate. Ambient relative humidity is about 40 percent. A potential of about 2500 volts is imposed on the roller electrode during exposure. After exposure, a full color image corresponding to the original is seen on the NESA surface. This image is of excellent quality, however, there is a noticeable variation in image density across the image surface.

EXAMPLE VII The imaging process of Example VI is repeated, except that sufiicient Sohio Odorless Solvent 3440 is applied to the roller electrode surface so as to fill the gap between the roller electrode and the NESA glass substrate before potential is applied across the electrodes. Again, the image produced is of excellent quality. Here, however, image density is uniform across the surface.

EXAMPLE VIII The imaging process of Example V1 is repeated, except that ambient relative humidity is maintained at about 85 percent. The image produced is of satisfactory quality; but with severe variations in image density across the image surface, giving a blotchy or mottled appearance to the image.

EXAMPLE IX The imaging process of Example VIII is repeated except that sufiicient Sohio Odorless Solvent 3440 is applied to the roller electrode surface so as to fill the air gap between the roller electrode and the NESA glass substrate before a potential is applied across the electrodes. The image produced is of excellent quality with very little variation in image density across the image surface.

EXAMPLE X The imaging process of Example IX is repeated, except that the liquid applied to the roller electrode surface to fill the air gap is decane, instead of Sohio Odorless Solvent 3440. Again, an excellent image is produced with little variation of density across the image surface.

Although specific components and proportions have been described in the above examples, relating to electrophoretic imaging, other suitable materials, as listed above, may be used With similar results. In addition, other materials may be added to the gap filling insulating liquid or to the particle suspension to synergize, enhance, or otherwise modify their properties. For example, the particle dispersions may be dye sensitized or electrically sensitized, if desired, or may be mixed or otherwise combined with other photosensitive materials, both organic and inorganic.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

1. The electrophoretic process comprising in combination the steps of:

(a) providing at least two electrodes, at least one of which is a blocking electrode movable relative to said other electrode.

(b) forming a suspension of photosensitive particles on a first portion of said olher electrode,

(c) filling a space between said blocking electrode and a second portion of said other electrode with an insulating liquid,

(d) subjecting said electrodes to an applied electric field, and

(e) simultaneously exposing said suspension to an image with activating electromagnetic radiation and moving said blocking electrode relative to said other electrode in contact with said suspension whereby an image is formed on at least one of said electrodes.

2. The process of claim 1 wherein said other electrode is a partially transparent injecting electrode.

3. The process of claim 1 wherein said blocking electrode is in the form of an endless surface which moves across said suspension in rolloing contact therewith.

4. The electrophoretic imaging process comprising the steps of:

(a) providing a pair of adjacent electrodes, at least one of which is at least partially transparent,

(b) forming a layer of a suspension of photosensitive particles on a first portion of one electrode,

(c) filling the space between a second portion of Said electrode and the other electrode with an insulating liquid,

(d) subjecting said electrodes to an applied electric field; and

(e) simultaneously exposing said suspension to an image with activating electromagnetic radiation whereby an image is formed on at least one of said electrodes.

5. The process of claim 4 wherein one of said electrodes is a blocking electrode.

References Cited UNITED STATES PATENTS 2,904,431 9/1959 Moncreitf-Yeates 961 2,975,052 3/1961 Folland et al. 96-1 GEORGE E. LESMES, Primary Examiner I. C. COOPER III, Rssistant Examiner U.S. Cl. X.R. 96-1, 1.3, 1.2, 1.4

53 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,485, 738 Dated December 23 1969 Inventor(s) L. M. Carreira It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 37, after "the" insert -system is the Column 4, line 12, delete "NEST" and insert NESA- Column 6, "J

t A line 30 after "comprising" insert -in combination-; line 32 delete "a pair of" and insert at least two SIGNED AND SEALED JUL 2 -1970 .sEAL

Attest:

Edward M. Fletcher, Jr. WILLIAM E. 50 Anesfing Commissioner of Paten 5 

